This invention relates to a gas dehydrating apparatus for gas controlled and powered systems, and more particularly to a membrane air dehydrating apparatus for use in pneumatically controlled and powered systems.
Compressed air systems are widely used to operate a number of devices, and find particular use in braking systems for commercial vehicles. These systems include an air compressor which compresses ambient air and charges a storage reservoir(s).
The air compressor is engine driven and sized to have a capacity to rapidly pressurize the reservoir(s) so that brakes are available as soon as possible after engine start, or for high duty cycles as would be required when descending winding mountain roads. Due to this, the air compressor has excess capacity during normal vehicle operation. To aid in fuel economy, the air compressor may be selectively decoupled (for example by a clutch) from the engine when the air pressure in the reservoir(s) is at a predetermined level. The control of the decoupling means is based upon reservoir pressure feedback.
Air dryers commonly employ a drying material or desiccant through which air passes to remove entrained moisture and limit potential problems stemming from moisture associated with brake system components. Moisture is adsorbed by the desiccant and removed from the compressed air before it is communicated to the storage reservoir. In these known systems, periodic or cyclic purging of the air dryers is desirable in order to purge or regenerate the desiccant.
International Publication No. WO 01/19657 discloses a trailer air dryer/moisture buffering system to separately protect devices that may not be protected by the tractor""s air dryer. This trailer buffering system allows the trailer braking and trailer suspension systems to be associated with the trailer dryer system, thereby avoiding contamination, the introduction of foreign particles, leakage, and freeze-up of the pneumatic valves. The disclosed trailer buffering system simultaneously filters and dries wet air sent to the trailer. This trailer buffering system uses an active drying material which may act as a moisture buffer, adsorbing moisture from the air during wet air consumption. The active drying material then slowly releases the previously captured moisture in vapor phase during a purge cycle, thereby regenerating the desiccant.
This type of buffering system is not an active air dryer, meaning the buffering system does not actively expel moisture from the air. For the supply air, a purge cycle could be incorporated with appropriate timing. Such a system generally would be rather complex and may be cost prohibitive.
With regard to the control air in this desiccant-type buffering system, a purge cycle could not simply be incorporated since it would be highly undesirable to interrupt the flow of control air, even for a very short time. Further, pressure swing adsorption (PSA) may be less effective with control air because it may consistently remain at rather low pressures. As such, in order to incorporate a purge cycle, the control air system would need two separate desiccant members, one for use and the other for purging, with appropriate timing for switching between the two. This added complexity renders the system cost prohibitive.
As may readily be appreciated, it is desirable to provide a gas dehydrating apparatus for gas controlled and powered systems, for example pneumatically controlled and powered braking systems for truck trailers, which continuously purges moisture removed from the control and supply gas. It is also desirable to provide such an apparatus which is relatively simple to manufacture and does not need electronically controlled timing and complex valving. Still further, it is desirable to provide such an apparatus which advantageously utilizes higher pressure supply gas to sweep extracted moisture from lower pressure control gas.
The present invention provides a method and apparatus for continuously removing moisture from a compressed air system. In one embodiment, the apparatus includes two membrane dryers acting in concert to dry a supply and a control gas for a pneumatically controlled and powered braking system.
In one specific embodiment, the air dryer system has a first housing having an inlet and an outlet and a flow path therebetween, and connected to a supply gas. A first membrane member is disposed within the first housing and in the flow path, and includes a plurality of tubes with interstitial spaces therebetween, each tube being more permeable to water vapor than to air. Supply gas passes through the first membrane member, thereby becoming substantially dehydrated. A portion of the dehydrated supply gas is channeled to the first membrane member interstitial spaces, whereby the dehydrated supply gas sweeps water and water vapor from the first membrane member interstitial spaces to the surrounding environment. A second housing has an inlet and an outlet and a flow path therebetween, and connected to a control gas. A second membrane member is disposed within the second housing and in the second housing flow path, and includes a plurality of tubes with interstitial spaces therebetween, each tube being more permeable to water vapor than to air. Control gas passes through the second membrane member, thereby becoming substantially dehydrated. A second portion of the dehydrated supply gas is directed from the first housing outlet gas to the second membrane member interstitial spaces, whereby the dehydrated supply gas sweeps water and water vapor from the second membrane member interstitial spaces to the environment.
Another aspect of the present invention is a method of drying compressed air used in a pneumatically controlled and powered braking system. In one embodiment, the method employs two membrane dryers that receive and dehydrate compressed air in a supply gas stream and a control gas stream.